Understand the definitions: A heterogeneous mixture has visibly different components or phases, while a homogeneous mixture has a uniform composition throughout.
Recall that air is composed of various gases like nitrogen, oxygen, carbon dioxide, and others, all mixed uniformly at the molecular level.
Since the gases in air are evenly distributed and you cannot distinguish separate parts by the naked eye, air is considered a homogeneous mixture.
Differentiate this from an element, which consists of only one type of atom, and a compound, which is a substance formed by chemically bonded atoms.
Conclude that air fits the description of a homogeneous mixture because it is a uniform physical blend of multiple gases without chemical bonding.